1. Field
The following description relates to a termination structure for a superconducting cable, and, for example, a termination structure for a superconducting cable for withdrawing a terminal of the superconducting cable from a side at a very low temperature to a side at room temperature.
2. Description of Related Art
As an existing termination structure for a superconducting cable, there is a termination structure disclosed in Korean Patent Publication No. 10-2007-102651. The existing termination structure for a superconducting is described with reference to FIG. 1.
Referring to FIG. 1, the existing termination structure for a superconducting cable includes a terminal of a superconducting cable 100, a bushing 10 which is connected to a superconductor 100a of the cable 100 to provide electrical conduction from a side at a very low temperature to a side at room temperature, a refrigerant container 11 for accommodating the terminal of the cable 100 and the bushing, a vacuum container 12 covering the outer periphery of the refrigerant container 11, and a porcelain tube 104 which protrudes and connects to a room temperature side of the vacuum container 12.
The porcelain tube 104 is filled with an insulating fluid 104a such as an insulating oil or an SF gas.
The refrigerant container 11 includes a liquid nitrogen layer (liquid refrigerant layer 13) and a nitrogen gas layer (gas refrigerant layer 14) on the very low temperature side and on the room temperature side, respectively, and the liquid nitrogen layer and the nitrogen gas layer are provided adjacent to each other. A flange 108 is interposed between the very low temperature side and the room temperature side.
The gap between an inner surface 11a of the refrigerant container 11 and the outer periphery of the bushing 10 is designed so that the nitrogen gas is not pressurized by a pressurizer but maintained in a gas state, and simultaneously, the pressure of the nitrogen gas and the pressure of liquid nitrogen are in equilibrium.
The bushing 10 (diameter 140 mm) includes the superconductor 100a of the superconducting cable 100, a conductor 10a (diameter 40 mm φ which allows electrical conduction, and a solid insulator 10b (thickness 50 mm) for coating the outer periphery of the conductor 10a. 
The superconductor 100a and the conductor 10a of the bushing 10 are connected by a joint 100b. The conductor 10a is made of copper having a relatively small electrical resistance in the vicinity of the temperature of liquid nitrogen.
The solid insulator 10b is made of an FRP having an excellent insulating property. In addition, as illustrated in FIG. 1, an upper screening portion 10c made of copper is provided at an upper end (an end portion positioned on the room temperature side) of the bushing 10.
The refrigerant container 11 is made of stainless steel, and is accommodated in the vacuum container 12 also made of stainless steel. A vacuum heat-insulating layer 11b is provided between the vacuum container 12 and the refrigerant container 11. The refrigerant container 11 includes the liquid nitrogen layer 13 and the nitrogen gas layer 14. A refrigerator 15 for refrigerating liquid nitrogen 13a is connected to the liquid nitrogen layer 13.
Before introducing the liquid nitrogen 13a into the refrigerant container 11, a nitrogen gas is charged in order to remove moisture and the like inside the refrigerant container 11. At the time of supplying the liquid nitrogen 13a to the liquid nitrogen layer 13, the nitrogen gas remains in the vicinity of a portion of the refrigerant container 11, specifically, in the vicinity of the flange 108 provided at the boundary between the very low temperature side and the room temperature side. As described above, the portion where the nitrogen gas remains becomes the nitrogen gas layer 14.
After the liquid nitrogen is introduced into the refrigerant container 11 to allow the pressure of the liquid nitrogen in the refrigerant container 11 to be about 0.5 MPa, the refrigerant container 11 is sealed. With such a configuration, a gas state is maintained only with the pressure the nitrogen gas, and the pressure of the nitrogen gas 14a and the pressure of the liquid nitrogen 13a are approximately in equilibrium.
The existing termination structure for a superconducting cable is divided into a room temperature section A, a temperature gradient section B, and a very low temperature section C. Since the conductor 10a included in the bushing 10 is formed integrally in such a structure, separation of the room temperature section A from the temperature gradient section B is impossible.
Accordingly, the existing termination structure only functions as an air termination connection box (connecting an overhead transmission line to an underground transmission line), and in the case where the terminal of the superconducting cable is connected to an external device such as a gas insulated transmission line (GIL) or a gas insulated switchgear (GIS), an additional air termination connection box is needed. Therefore, the structure of an apparatus becomes complex, a large area is needed, and an insulating weakness portion of the apparatus increases.
In addition, sealing members made of insulating materials are provided to seal the sections at the inner boundary portions between the room temperature section A, the temperature gradient section B, and the very low temperature section C. Since this sealing member is the most vulnerable portion, when an insulation failure occurs in the sealing member at one of the boundary portions, there is a problem in that the entire bushing has to be replaced.